Black holes balance the books when gobbling mass

An astrophysical balancing act occurs in one of the brightest X-ray sources …

Supermassiveblackholes are often accompanied by two things: an accretiondisk of matter in its death throws and a pair of relativisticjets that eject some of this matter in a last-minute pardon prior to its removal from the universe. While black holes are known for capturing every bit of matter or energy that gets too close, it has also been theorized that they will eventually stop growing. It is believed that these two phenomena—matter being devoured at the edge of the disk while being pushed away by the jets—form a sort of self-regulating system that keeps these monsters in check.

While this self-regulatory behavior is believed to be common in supermassive black holes, the extreme mass scales involved mean that the dynamics of the system occur on extremely long time scales, on the order of tens of thousands of years. However, if the black hole is smaller—on the order of a few solar masses—then the dynamics of these phenomena should operate on more earthly timescales, such as a few hours. It's worth checking, because if these phenomena are not detected in smaller black holes, then physics has a big problem on its hands, since the same equations describe both sizes of black holes.

Using seven years of observations by the Chandra X-Ray observatory, a pair of astronomers from Harvard have shown that the same physics hold true in both cases. As described in the current edition of Nature, they studied GRS 1915+105, a stellar mass black hole complete with relativistic jets. This object can be described as a microquasar, a small-scale version of an active galactic nucleus.

The astronomers have identified a dynamic balancing act taking place between the relativistic jets and a hot wind blowing off the accretion disk. GRS 1915+105 is a 14 solar mass black hole that is accompanied by—and is eating—a 0.8 solar mass star that orbits it every 33.5 days. Early observations of GRS 1915+105 found it to be something special: it was the first object ever identified to have matter leaving it with speeds that appear faster than light.

Given its significance, many observatories have focused on it and researchers have found over 14 distinct classes of energetic output thought to be the result of different types of disk-jet interactions. Using data from April 2000 through August 2007, the astronomers were able to observe five of the 14 classes of X-ray emissions, and were able to lump them into two general states—hard and soft. In the 11 distinct observations of GRS 1915+105 that were made over this time period, the duo identified five observations of the various bright, soft states, and six of the faint, hard-jet-producing states.

Using an examination of the emission spectra of GRS 1915+105 during these different states, the authors postulate that a broad Fe XXV emission line is produced when the "hard" X-rays hit the inner edge of the accretion disk. Through calculations, they were able to deduce the position of the inner edge of the accretion disk during this phase, placing it around 250 times the Schwarzchild radius of the black hole. During the bright, soft states, the inner edge of the accretion disk may lie as close as three Schwarzchild radii to the black hole.

Through these observations, the pair concludes that these emission lines originate in an accretion disk wind. For a wind to occur, some driving force must exist behind it, giving it a push. The obvious candidate in such a situation would be radiation pressure, but astrophysical calculations show that radiation pressure alone would to insufficient to impart enough momentum to drive this wind. The remaining push can, however, come from X-Ray heating and thermal pressure. Sample calculations confirm that a thermal driving force, assisted by radiation pressure, can successfully produce this mighty wind.

The researchers calculated that the wind is capable of carrying away approximately 10-8 solar masses worth of material each year. The rate at which the wind drives mass away from the black hole is interesting, because it is approximately the same rate of mass driven away via the relativistic jets. This is doubly interesting because it suggests that the black hole is able to maintain a balance of mass coming in and mass going out, regardless of the mechanism by which mass leaves the system—jet or wind.

The authors conclude by stating that these observations give "a strong indication that like their supermassive counterparts, stellar-mass black holes can regulate their accretion rate by feedback into their environments." They also reiterate the importance of the finding that the high intensity of the radiation field from the disk is able to re-direct the accretion flow away from the relativistic jet, and into the outward bound wind: "our results point to fundamental new insights into the long-term disk-jet coupling around accreting black holes and hint at tantalizing evidence of the mechanism by which stellar-mass black holes regulate their own growth." This reassures us that the equations that describe gravity do indeed work at the vastly different length and time scales that exist between stellar mass and supermassive black holes.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.